Ion Mobility Spectrometry (IMS) is a technique that separates ions in terms of their mobility with reference to a drift/buffer gas. The analysis is based on measuring the velocity which gaseous ions attain while drifting a defined distance through the buffer gas. Prior art mobility techniques are known as “time-of-flight” separation techniques if based on time or as “differential” separation techniques if ion detection is based on position. Drift-tube mobility analyzers operate by recording the time-of-flight of ion packets separated by their mobility in a background gas under the influence of a uniform electric field. The background gas is introduced at a low flow rate in the opposite direction.
A differential mobility analyzer (DMA) operates by combining the perpendicular forces of a flowing gas and an electric field. The diffusion limited resolution attainable with a TOF analyzer is substantially higher than typical DMA values but DMAs attain higher sensitivities by virtue of the continuous ion beam analyzed (a factor of ˜100 because of the low duty cycle of TOF analyzers). One disadvantage of TOF mobility analyzers is that the particles/ions are not easily selected according to their mobility. Additional ion gates can be employed but aliasing of the mobility due to multiples of the period and the low resulting duty cycle make that type of instrument unreliable and inadequate for mobility selection. In contrast, a DMA instrument is naturally useful for mobility selection providing a continuous stream of ions of one size that can then be directed toward other instruments/analyzers i.e. mass spectrometer, optical spectrometer, ion trap, surface (soft-landing, dissociation), etc.
There are three factors that adversely influence the motion of ions in a prior art DMA: (1) dilution by mixing of the inlet flow and sheath gas flow, (2) ion diffusion (Brownian motion), and (3) finite slit width
A theoretical value of resolution (R) calculated as a Gaussian-type transfer function with full width ΔZ at half-height can be approximated as a summation of the square of the variances due to these three factors as in Equation 1
                                          (                          Z                              Δ                ⁢                                                                  ⁢                Z                                      )                    2                =                                            (                                                Q                  sh                                                  Q                  in                                            )                        2                    +                      Pe                          16              ⁢                                                          ⁢              ln              ⁢                                                          ⁢              2              ⁢                              (                                  b                  +                                      b                                          -                      1                                                                      )                            ⁢                              G                ⁡                                  (                                      y                    i                                    )                                                              +                                    (                              L                                  2                  ⁢                  Δ                  ⁢                                                                          ⁢                  L                                            )                        2                                                        Equation          ⁢                                          ⁢          1                ⁢                                      
where Z is the particle/ion mobility, Qsh is the sheath gas flow rate, Qin is the inlet gas flow rate, G(yi) and b are dimensionless geometry factors, Pe is the Peclet number (a dimensionless engineering parameter related to x, y, z), L is the axial length from entrance to exit slit, and ΔL is the exit slit width (See H. Tanaka, K. Takeuchi, Aerosol Science, (2003) vol. 34, 1167-1173).
To obtain higher resolution, a DMA should be operated at higher gas flow velocities (high Reynolds or Peclet numbers) and at subsequently higher voltages (first terms on the right, and the Z/ΔZ term, in Equation 1, respectively) (See J. F. de la Mora, L. de Juan, T. Eichler, J. Rosell, Trends in Anal. Chem., (1998) vol. 17, 6, 328-338). In turn, the pumps required to obtain high gas flow velocities become larger as the inlets are better designed to create laminar flow. Typical commercially available aerosol DMAs can obtain a resolution from 1 to ˜10. For example, the TSI Inc. Particle Sizer Spectrometers operate with (Qsh/Qin) equal to 4, limiting the resolution to <4. (Model 3091 Fast Mobility Particle Sizer Spectrometer, 2004; Model 3034 Scanning Mobility Particle Sizer, 2003) Higher performance research instruments of “unusually high resolution”, in practice typically operate at a resolution <25 because of high inlet flow rates and extremely narrow (<0.1 mm) exit slits requiring pumping speeds up to 3000 l/min. These factors limit the resolution of prior art DMAs severely and the highest reported resolution of 60 required a Reynolds number approaching 100000. These pumping requirements are impractical; therefore a new DMA design is needed.
FIG. 1 shows a schematic of a prior art DMA common to the field in which mobility separation is obtained by using two planar plates (A and B) between which a laminar gas flow (Qsh), and a scanning voltage (E) are applied. Charged particles/ions for analysis are introduced with Qin and are detected by an electrometer. The operating principle and range of use is basically identical to that of the cylindrical DMA, the difference being only that the electric field is uniform between the two plates whereas the field inside the cylindrical DMA is increasing (in absolute value) toward the center electrode. Various attempts have been made to improve DMA performance including inversion of the direction of particle paths and reductions of the length to gap ratio. In particular, a number of attempts have been made to vary the electric field configuration with respect to the gas flow following a design originally proposed by Loscertales. (See Loscertales, “Drift Differential Mobility Analyzer”, Journal of Aerosol Science, 1998, 29, 1117-1139) These attempts include the “Inclined Grid” method of Tammet (See Tammet, “The Limits of Air Ion Mobility Resolution”, Proc. 11th Int. Conf. Atmos. Elect., NASA, MSFC, Alabama, 1999, 626-629), the various configurations of Labowsky and De La Mora (WO2004/077016 A2), the “Cross-Flow Differential Migration Classifier” of Flagan (U.S. Pat. App. 2004/0050756 A1), and the cylindrical “Cross-Flow Ion Mobility Analyzer” of Rockwood et. al. (U.S. Pat. App. 2005/0006578 A1) All of these methods change the geometry factor of cylindrical or planar devices and the increase in resolution is determined by the amount of electrical work done with respect to the gas flow but also by the other factors in equation 1. Those factors may limit or even decrease the overall resolution. In practice the only significant improvement in resolution has involved increasing the gas flow rate, an exception being the device of Gillig et al., a Multislit-Multigap DMA (MMDMA) consisting of a series of DMAs where the ions pass through a series of small slits, reducing the pumping requirements and improving the resolution by maintaining laminar flow at high gas velocities. (See K. J. Gillig, R. Sperline, M. B. Denton, Multi-slit/Multi-channel Differential Mobility Analyzer, Pittcon 2007, Chicago, Ill., USA) A schematic of a MMDMA is shown in FIG. 2.
There is a continuing need for methods to reduce the pumping requirements in ion mobility spectrometry and differential mobility analysis. There is also a need for an apparatus and arrangement for mobility spectrometry and analysis to obtain high resolution and identification of analytes.